Even though there are still some discussions on that, Luke is widely recognized as the author of the book of Acts. Wikenhauser-Schmid (Einleitung, 376) list the following modern interpreters who recognize that: Dibelius, E. Meyer, T. W. Manson, Streeter, von Campenhausen, Eltester, Trocmé, van Unnik, Klijn, McNeile, and Bauernfeind.
I wonder, however, why is there a separation between the gospel of Luke and the book of Acts, something that tends to obscure the second-volume character of Acts?